Over the last several years, radio-communication technology, and in particular cellular phone technology, has become a fixture in the everyday lives of many Americans. No longer tied to the fixed public switched telephone network, people have grown accustomed to the great flexibility and freedom that this "wireless" technology allows.
However, there still remains the annoying necessity of holding the cellular phone unit in place during use. As a consequence, the user either has to keep at least one hand holding the unit or has to crook his or her neck in an attempt to support the unit between the ear and the shoulder. As cellular phones become smaller and smaller, the later option becomes less and less viable.
To eliminate this problem, it has been suggested that cellular phones be equipped with circuitry to allow the phone to be used in combination with a headset which the cellular phone user can wear. Having one or more headphones and a microphone, as well as a series of control switches to turn the headset on and off, these headset/phone systems offer a tantalizing "hands free" alternative to those cellular phone users who seem to have their cellular phones "glued" to their ears.
To offer such an alternative, the headset/phone system must be able to establish a communications link between the headset and the phone. The link must support full duplex transmission of control information and audio information, with the control information preferably in digital form and the audio information presently preferably in analog form.
At the present time, at least one headset/phone system has been developed which provides duplex transmission of control and audio information. The system employs bursts or pulses of an RF carrier to convey control and audio information. Specifically, the time between pulses is used to convey both the control and audio information.
This system has a number of drawbacks. The signal to noise ratio may be poor, because of a narrow range of timing values employed between pulses. Moreover, all headset/phone pairs of this system operate on the same frequency, the manufacturer having attempted to use time shifting to separate the pulse streams of different headset/phone pairs. As a consequence, if two or more headset/mobile phone pairs of this system are used in close proximity over a prolonged period of time, problems may eventually occur as the time bases of different units "drift" into an interference condition.